This invention relates to a multiplex transmission system for vehicles, in which multiplex transmission such as of the CSMA/CD type is applied to the transmission of signals in an automotive vehicle. More particularly, the invention relates to a signal transmission system in vehicles, which system is so adapted that a plurality of decentralized-type multiplex communication networks are formed and the networks are interconnected by gateway nodes.
The greater use of electronics in automotive vehicles has been accompanied by a number of serious problems, among which are the increased size and complexity of the wire harnesses that interconnect the electronic components. Multiplex communication has come to the fore as a means of solving these problems, particularly in the automobile field. Multiplex communication refers to transmission, by time-division multiplexing, of plural items of data on a single wire harness, and basically employs serial transmission.
In the field of automobiles, multiplex communication networks may be considered as being divided into either of two classes, one class being complete multiplex- and partial multiplex-type networks, and the other being centralized- and decentralized-type networks.
The partial multiplex-type network mixes a non-multiplex communication portion and a multiplex communication portion. In the multiplex communication portion, switches and loads decentralized in terms of distances are connected by a multiplex transmission unit. Since separate wiring is required between the multiplex transmission unit and the switches and loads, a drawback is that the numbers of wires is greater, although the overall length of the interconnecting wiring is reduced.
The centralized-type network is one in which a plurality of slave transmission units are connected to a single master transmission unit. With a network having this configuration, a smaller harness diameter is achieved but the entire system is rendered inoperative if the master transmission unit fails. Another shortcoming is that difficulties are encountered in design modification. On the other hand, the decentralized network, though higher in cost, exhibits a number of advantages, such as a much smaller harness diameter, high reliability with regard to partial failure and greater flexibility in terms of design modification. (For an example of the latter, see the specification of Japanese Patent Application Laid-Open No. 62-4658).
With regard to this decentralized multiplex communication system, a CSMA/CD system is employed in accordance with an SAE (Society of Automotive Engineers) standardization proposal.
The assignee of this invention has also proposed a PALMNET (protocol for automotive local area network) system which is a further development upon this CSMA/CD system. For example, see the specification of Japanese Patent Application No. 62-302421.
Further, in the specification of Japanese Patent Application Laid-Open (KOKAI) No. 64-36541 belonging to the assignee of this invention, a master node transmits a reference pulse for time sharing the right to access a bus in order to set a channel on a transmission line.
In recent multiplex communication systems for vehicles, it has been proposed to connect vehicle controllers (an engine controller, traction controller, etc.) by multiplex communication. In these controllers, data to be transmitted to other controllers is generated quite frequently, and so is data necessary for these controllers themselves. Therefore, in a case where communication nodes for such high-speed controllers are mixed with nodes for low-speed electrical accessories, such as actuators (motors, etc.) and sensors, the communication speed of the overall system must be made high or faster. This increasing of the communication speed for the overall system is a cause of higher cost.
With the greater sophistication of traction control, rear-wheel steering control and ABS control, etc., not only is a higher processing speed necessary for such control, but the information data employed by these controllers is closely interrelated with the controllers. In other words, a highly frequent exchange of information among the controllers is essential. More specifically, the time has come to reconstruct the system by treating nodes for controllers having a high degree of interrelationship as being separate from other ordinary nodes, and considering them as one group.
When reconstructing the network, an important consideration is the degree to which the reconstruction will contribute to control of vehicle travel. For example, the fact that the nodes for controllers are closely interrelated should be taken into consideration.